In recent years, with the rapid spread of a small electronic equipment such as a mobile phone or a notebook-sized personal computer, demand for a non-aqueous electrolyte secondary battery which is used as a chargeable and dischargeable power supply has been rapidly increased. As a positive electrode active material for a non-aqueous electrolyte secondary battery, lithium-nickel composite oxide represented by lithium nickel dioxide (LiNiO2) and lithium-manganese composite oxide represented by lithium manganese dioxide (LiMnO2) have been widely used as well as lithium-cobalt composite oxide represented by lithium cobalt dioxide (LiCoO2).
However, there are some defects in the lithium cobalt dioxide, such that the lithium cobalt dioxide is expensive because its reserve is a little in the earth, and that the lithium cobalt dioxide contains cobalt which is unstable in supply and has a highly fluctuating price range as a major component. Therefore, there have been remarked lithium-nickel composite oxide containing relatively inexpensive nickel as a major component and lithium-manganese composite oxide containing relatively inexpensive manganese as a major component from the viewpoint of reducing in costs. The lithium manganese dioxide is superior in thermal stability to lithium cobalt dioxide. However, the lithium manganese dioxide has some problems in practical use in a battery, because its charge and discharge capacity is much smaller than that of the other materials, and its charge and discharge cycle characteristic showing life of a battery is also much shorter than the other materials. On the other hand, since the lithium nickel dioxide has a charge and discharge capacity greater than the lithium cobalt dioxide, the lithium nickel dioxide has been expected to be used as a positive electrode active material which enables to produce an inexpensive battery having a high energy density.
The lithium nickel oxide has been usually prepared by mixing a lithium compound with a nickel compound such as nickel hydroxide or nickel oxyhydroxide, and calcining the resulting mixture. The form of the lithium nickel oxide is a powder in which primary particles are mono-dispersed or a powder of secondary particles formed by aggregation of primary particles and having spaces between the primary particles. However, both powders have some defects such that the powders are inferior in thermal stability under the condition of charging to the lithium cobalt dioxide. In other words, since pure lithium nickel dioxide has defects in thermal stability, charge and discharge cycle characteristics and the like, the lithium nickel dioxide cannot be used in a practical battery. This is because stability of the crystal structure of the lithium nickel dioxide is inferior to that of the lithium cobalt dioxide under the condition of charging.
Therefore, in order to stabilize crystal structure under the condition in which lithium is eliminated, and to obtain lithium-nickel composite oxide having favorable thermal stability and charge and discharge cycle characteristics as a positive electrode active material, there has been generally carried out replacement of a part of nickel contained in lithium-nickel composite oxide with other substance. For example, there has been carried out replacement of a part of nickel with a transition metal element such as cobalt, manganese or iron, or a heteroelement such as aluminum, vanadium or tin (see, for example, Patent Literature 1).
In addition, as a process for improving thermal stability of lithium-nickel composite oxide, there has been developed a process for washing lithium nickel dioxide with water after calcining (see, for example, Patent Literatures 2 and 3). When the lithium nickel dioxide is washed with water after calcining, it is thought that there can be obtained from the lithium nickel dioxide a positive electrode active material which has a high volume, and is excellent in thermal stability and preservation characteristics under high temperature circumstances in the case where the positive electrode active material is used in a non-aqueous electrolyte secondary battery.